The continuous casting of metal in a peripheral groove around a rotating casting wheel is well known in the metal foundry art. In the casting of metal in these rotating casting wheels, it has been found that the metal solidifies in three distinct phases as it cools. The first phase begins when the liquid metal is fed into the peripheral groove of the casting wheel and includes that portion of the casting process during which the metal is cooled but is completely liquid within the casting wheel so as to be in complete contact with the casting wheel. The second phase is that portion of the casting process during which the continued cooling of the metal causes an outer crust of solidified metal to form adjacent the casting wheel but during which the metal is still in substantially complete contact with the casting wheel. The third phase is that portion of the casting process beginning generally at or near the point in the solidification of the molten metal at which the continued cooling of the metal and the thickening of the outer crust of solidified metal causes the metal to shrink away from the casting wheel and form an air gap between the metal and the casting wheel. Thus, the third phase includes that portion of the casting process during which the air gap prevents complete contact between the hot metal bar and the casting wheel. The metal bar may not be completely solidified and therefore requires further cooling.
It is this third phase of solidification that is most troublesome in the casting of molten metal in prior art rotating casting wheels since the air gap formed between the cast metal and the casting wheel greatly reduces the rate of heat transfer from the metal to the casting wheel. This is because the heat must be transferred from the cast metal to the casting wheel, in the third phase, principally by radiation heat transfer through the air in the gap between the cast metal and the casting wheel rather than by conduction heat transfer as in the first and second solidification phases, and because less heat can be transferred by radiation heat transfer than by conduction heat transfer at the same relative temperatures.
The low rate of heat transferred during the third phase of solidification in a prior art casting wheel in turn results in limiting the maximum rotational speed of the casting wheel, hence limiting the casting rates that can be achieved. This is because the rotational speed of a prior casting wheel must be slow enough to provide a sufficient dwell time of the metal in the casting wheel during the third phase for the metal to solidify sufficiently in the casting wheel, and because the length of the arcuate casting mold available for the third phase of solidification is limited by structural considerations.
This serious limitation of the maximum casting rate has been recognized in some prior art attempts to increase the cooling of the cast bar during its last phase of solidification. However these attempts are generally not successful in actual practice due to the complex apparatus which often fails to work under the harsh conditions of industrial production. The methods and apparatus disclosed in U.S. Pat. Nos. 3,261,059 and 3,575,231 are exemplary of this prior art.
These patents essentially disclose the use of multiple rollers or wheels for guiding and holding the band away from the casting wheel so that a fluid can be forced entirely around the hot cast bar, totally filling the solidification gap, functioning either as a heat-transfer medium to conduct heat across the gap to the walls of the mold, or as a direct coolant medium to directly cool the peripheral surfaces of the cast bar.
However, not only do these methods fail to achieve the same degree of cooling that can be achieved by direct contact between the cast bar and the walls of the casting groove, but by removing the band from contact with the cast bar and permitting the bar to drop downwardly out of the casting groove so that the fluid can be caused to flow entirely therearound, the bar is no longer firmly supported by the walls of the mold. Consequently, under these conditions, the internal stresses in the still-soft cast bar tend to cause it to deform or even crack, thus adversely affecting the quality of the cast product. Moreover, the use of rollers to deflect the band away from the periphery of the casting wheel induces additional stresses in the band which adversely affects its useful life. Furthermore, in actual practice, these rollers often become inoperable due to an accumulation of metal spilled during the casting operation.